The present invention relates to a filter for a cigarette and a filter cigarette using the filter.
A filter of this type is known which has a plurality of axial holes therein extending from one end of the filter to the other. The axial holes are formed by tubes embedded in the filter (see U.S. Pat. No. 3,546,325) or by thermoforming. The thermoforming is carried out by melting part of the filter using a laser beam (see U.S. Pat. No. 4,291,712) or a pin-like heating tool.
Such a filter makes it possible to introduce part of the main smoke stream from the cigarette directly and without filtering treatment into the smoker""s mouth.
In recent years, many smokers have shown a tendency to like a milder smoking feeling without loss of enjoying an original cigarette aroma or flavor. Therefore, it is necessary to optimally control the amount of smoke stream which is introduced into the smoker""s mouth through the axial holes of the filter; the axial holes of the prior filters, however, deliver the smoke stream from the cigarette directly into the smoker""s mouth, and there is therefore a limit to the extent to which the smoking feeling can be lightened.
A cigarette filter according to the invention comprises a filter rod which has an air permeable peripheral surface and in which 3 to 12 continuous passages extend between open ends at the two ends of the rod, the passages being distributed on a ring having a diameter of 50 to 70% of that of the rod, and each the passages having a diameter of 0.1 to 0.7 mm and the wall of each said passage having air permeability.
The filter rod which has an air-permeable peripheral surface may for example be a body of filtering material held in rod form by an air-permeable circumferential wrapper; it might instead be a filter rod which is coherent and self-supporting and dimensionally stable without a circumferential wrapperxe2x80x94examples of this being made of fibers or filaments bonded at points of contact into permeable rod form, such as the commercially available NWA filter rods of bonded cellulose acetate filaments. Where the filter rod does not itself need a wrapper to maintain its rod configuration, it may nonetheless be provided with a ventilating wrapperxe2x80x94e.g. one which joins it to a longitudinally adjacent rod to form a composite filter, and/or a ventilating tipping overwrap which joins it to a tobacco rod to form a filter cigarette.
The filter is usually connected to a cigarette through tipping paper having air permeability. When the filter cigarette is smoked, ventilation air is introduced into the rod through the tipping paper and then into the passages through theirwalls. The smoke stream which passes through the passages is therefore diluted by the ventilation air for delivery to the smoker. With the diameter and the number of the passages limited as above, the amount of the main smoke stream diluted by the ventilation air is optimal to give a mild smoke while providing the smoker the original taste and aroma of the cigarette.
The passages may be made by heating the filter material with vapor so as to thermoform the filter material without melting.
Preferably, each of the passages has a diameter of 0.2 to 0.5 mm. The axial air-flow resistance of the filter, that is, resistance to draw (RTD) through the filter, is preferably 80 to 160 mmH2O.
The filter may include, at an end thereof, a tip having a length of 2 to 20 mm which is correspondingly 8 to 60% of the total length of the filter, and having RTD of 80 or less mmH2O/25 mm; a tip e.g. a plain filter tip makes it easy to adjust the RTD of the whole filter.
The amount of ventilation air introduced radially into the filter is preferably 20% or more, more preferably 70 to 80%, of the total amount of air flowing into the filter. By adjusting the amount of ventilation air, the amount of tar delivered to the smoker through the filter can be determined.